The present invention relates to a laser beam recording apparatus, such as a laser beam printer, wherein a photosensitive member is scanned with a laser beam which is modulated in accordance with an image information to be recorded.
Conventional laser beam printer will first be described in conjunction with the accompanying drawing, FIG. 1.
The laser beam printer includes a photosensitive member 1, which is in the form of a drum, rotatable in the direction shown by the arrow. Around the photosensitive drum 1, there are provided a charger 2 for uniformly charging the surface of the photosensitive drum 1 and developing means 3 for developing an image on the photosensitive drum 1. The printer further includes a paper feed cassette 4, registration roller 6, an image transfer charger 7, image fixing means 8 and a cleaner 9.
The laser beam printer is provided with a housing 10 for accommodating an optical system for the laser beam L. The housing 10 contains a laser beam oscillator 11 which generates a laser beam L modulated in accordance with external image signals, a polygonal mirror 12 for deflecting the modulated laser beam L to scan therewith the photosensitive drum 1 in the direction of the rotational axis, a driving motor 13 for rotating the polygonal mirror at a constant rotational speed, an f.multidot..theta. lens 14 for imaging the deflected laser beam L on the surface of the photosensitive drum 1, and a reflection mirror 17 for deflecting the optical path.
The housing 10 is provided, partly opposed to the photosensitive drum 1, with an opening for allowing the laser beam L to emerge. The opening is sealed by a transparent parallel-plane plate, such as a glass plate, which functions as a dust proof plate for protecting the laser beam optical system within the housing 10 from the dust.
Further, a filter 16 may be disposed across the optical path of the laser beam L, when, for example, the amount of light is to be controlled.
In operation, the laser beam L modulated in accordance with the external image information signals, scans the photosensitive member, i.e., drum 1 at its surface which has been uniformly charged by the charger 2. By this scan, an electrostatic pattern is formed on the photosensitive drum 1 in accordance with the image signal. The electrostatic pattern is then visualized by the developing means 3. The visualized image is then transferred by the transfer charger 7 onto a transfer material fed from the feed cassette 4. The transfer material bearing a visualized image thereon is conveyed to the fixing means 8, whereat the image is fixed thereon. The transfer material is then discharged out. On the other hand, the residual developer remained on the photosensitive drum 1 is removed therefrom by cleaning means 9 to make it ready for the next image formation.
It has been found that the resultant images produced by the above-described apparatus sometimes have unintentional stripe patterns.
The inventor has been making detailed investigation on this problem, and finally found that the cause of this is the interference of the laser beam, which takes place in the parallel-plane plate of uniform thickness, such as the glass plate 15 and the filter 16, disposed across the optical path.
The cause which was found as a result of the investigation will be explained in detail in conjunction with FIG. 2, wherein the parallel-plane plate 15 (or 16) which is transparent for the laser beam L is shown as viewed in the direction of A for the dust proof glass plate 15 (or B for the filter 16). The laser beam L, which has been deflected by the polygonal mirror 12 (not shown in this Figure), is transmitted through the parallel-plane plate 15 (or 16) to the photosensitive drum 1. The laser beam L moves scanningly. In FIG. 3, the laser beam LA is shown as that at a certain instance. The laser beams LB and LC are shown as at the next instances in this order. The beams LA, LB and LC are transmitted through the plate 15 (or 16) as LA1, LB1 and LC1. Additionally, a part of each of the beams LA, LB and LC is reflected by the opposing surfaces of the plate 15 (or 16) and then transmitted therethrough as shown by reference characters LA2, LB2 and LC2. The beams LA1, LB1 and LC1 are interfered with LA2, LB2 and LC2, respectively due to the optical path length difference therebetween. When the optical path length difference between the transmitting beam LB1 and the reflected beam LB2, for example, is m.lambda. (m is an integer, and .lambda. is the wavelength of the laser beam), those beams are strengthened. When, on the contrary, the optical path difference is (2 m+1/2).lambda., they are compensated.
When the laser beam deflects from the position shown by the beam LA to the position shown by the beam LC, the transmitting beam and the reflected beam are strengthened and compensated repeatedly to form a stripe pattern on the photosensitive drum 1.